A vinyl chloride-based polymer is a synthetic resin which is used the most among thermoplastic resins. In a polymerization method of a vinyl chloride-based polymer, there are suspension polymerization, emulsion polymerization, and bulk polymerization. Among these, the bulk polymerization is a method in which only a vinyl chloride-based monomer, an initiator, and, as necessary, a reaction additive are supplied and polymerized without use of water and a dispersant (emulsifying agent). The bulk polymerization has advantages in that a device is simple, a reaction is rapid, a yield is high because a purification process comprising distillation, extraction, and the like is not undergone, a polymer with high purity can be obtained, and the polymer can be handled without change.
However, the bulk polymerization has a disadvantage in that it is difficult to adjust a temperature due to generation of a large amount of heat in the polymerization. Also, the bulk polymerization has a disadvantage in that it is difficult to diffuse heat of reaction by conduction or convection because there is no material which is capable of absorbing and removing heat of polymerization other than a vinyl chloride-based monomer and viscosity of a polymerization material increases as the polymerization is progressed. Accordingly, a vinyl chloride-based polymer prepared by the bulk polymerization may be thermally damaged due to heat of reaction generated in a bulk polymerization process or unexpectedly generated heat. Therefore, it is very important to ensure thermal stability of a vinyl chloride-based polymer.